1) Field of the Invention
The present invention relates to a technology for electric discharge surface treatment, more particularly to an electrode for electric discharge surface treatment.
2) Description of the Related Art
For a turbine blade of a gas turbine engine for aircrafts, it is necessary to process a surface of the turbine blade by coating or hardfacing with a material that has strength and lubrication property in high-temperature environments. Chromium (Cr) and molybdenum (Mo) become lubricant if Cr and Mo are oxidized in a high-temperature environment. Therefore, a cobalt (Co) base material that includes Cr or Mo is used to form a thick coat through a scheme, such as welding or thermal spraying.
In the welding, a material of the welding rod is fused to deposit on a work by causing electric discharge between the work and the welding rod. In the thermal spraying, a metallic material is melted, and a coat of the metallic material is formed on a surface of a work by spraying the metallic material melted on the surface.
However, both the welding and the thermal spraying are manual operations and require a skill. Therefore, it is difficult to perform such operations on a production line, disadvantageously leading to an increase in a manufacturing cost. Moreover, since the welding in particular is a scheme in which heat intensively enters a work, if a material to be processed is thin or easily broken, weld cracking is likely to occur, thereby disadvantageously reducing yield. A material that is easily broken is, for example, a single-crystal alloy or a directionally-controlled alloy, such as a unidirectionally-solidified alloy, is processed.
As another technology for surface treatment, a surface treatment by electric discharge machining (hereinafter, “electric discharge surface treatment”) has also been established. Such a technology is disclosed, for example, in International Publication No. 99/58744 Pamphlet.
In forming a thick coat by the electric discharge surface treatment, the most important factors that influence coating performance are supply of a material from an electrode, and a condition of melting and bonding with a work material of the material supplied on a work surface. The strength, that is, hardness, of the electrode has an influence on the supply of the electrode material. In a method disclosed in International Publication No. 99/58744 Pamphlet, with an electrode for electric discharge surface treatment that is hard to some extent, supply of an electrode material by electric discharge is suppressed, and the electrode material is sufficiently melted to form a hard ceramic coat on a work surface. However, the coat is limited to a thin coat of up to approximately 10 micrometers (μm).
Therefore, it is impossible to form a dense thick coat (as thick as 100 μm or more) to be used for purposes requiring strength and lubrication property in the high-temperature environments as described above.
In a conventional electric discharge surface treatment, a green compact formed by compression molding a ceramic powder is used to form a coat of a hard material, such as titanium carbide (TiC), to improve abrasion resistance of a component and a mold. An electrode that is used in such electric discharge surface treatment is manufactured by compression molding a ceramic powder with a press and then by heating. Such a technology is disclosed in, for example, Japanese Patent No. 3227454.
In recent years, demands for forming a metallic coat that has a lubrication property and corrosion resistance by electric discharge surface treatment are increasing. It has become apparent by experiments conduced by the inventors that, to form a metallic coat having a lubrication property and corrosion resistance by electric discharge surface treatment, a metallic powder of which an average grain diameter is 3 μm or less is required to be used to manufacture an electrode.
However, grains in such metallic powder tend to coagulate into large solids due to a strong force occurring between the grains by an action of an intermolecular force or an electrostatic force. If an electrode of a green compact that is formed with the metallic power including such large solids is used in the electric discharge surface treatment, the large solids are deposited on the work surface, thereby disadvantageously causing not only a short circuit and instability in electric discharge but also deterioration in surface roughness of the coat.
In a technology disclosed in the invention of Japanese Patent No. 3227454, a ceramic powder, in which a force between grains thereof is weak, is used. Therefore, the grains are less likely to coagulate into large solids even after mixing paraffin. Thus, in the invention of Japanese Patent No. 3227454, no measure is taken for addressing such coagulation.
Moreover, conventionally, in manufacturing a metallic electrode, another technology for manufacturing an electrode that is not formed with the green compact has been established. In the technology, the electrode is formed by molding a metallic powder is molded by a press, and then by heating until metal of the metallic powder is completely melted. Also in this case, since the metal is melted, no measure is taken for addressing the coagulation of grains of the metallic powder.
Furthermore, in a conventional method for manufacturing an electrode, an electrode is manufactured by compression molding a commercially-available ceramic powder with a press in an atmosphere, and then by heating (for example, the method disclosed in Japanese Patent No. 3227454). Ceramics used for the electrode has a high oxidation temperature. Therefore, even if a dried powder of ceramics having an average grain diameter of the order of 1 μm is left in an atmosphere, ceramics is not oxidized. Thus, it is easy to prepare a material because a ceramic powder having an average grain diameter of several μm is commercially available. In addition, molding is easily performed.
Furthermore, another method for electric discharge surface treatment is disclosed in Japanese Patent Laid-Open Publication No. H5-148615. In the method, a thick coat of which thickness is several tens of millimeters (mm) is formed with tungsten carbide (WC) and Co each having an average grain diameter of the order of 1 μm. WC and Co are also metal less likely to be oxidized as well as TiC. Such metal less likely to be oxidized includes nickel (Ni). Such technology for forming a coat of hard ceramics on a work surface by an electrode formed with ceramics and WC has been achieved by conventional technologies.
As described above, in recent years, there have been increasing demands for a metallic coat having a lubrication property and corrosion resistance in high-temperature environments that are formed by the electric discharge surface treatment. Moreover, for maintenance and dimensional correction of metallic components, application of a thick coat formed with metal or an alloy by the electric discharge surface treatment has been demanded. As described above, it has become apparent that, to form a coat of metal or an alloy by the electric discharge surface treatment, an electrode is required to be manufactured with a powder having an average grain diameter of 3 μm or less.
However, a metallic powder or an alloy powder having a grain diameter of 3 μm or less that are available on the market are limited to powders of material less likely to be oxidized. In other words, it is difficult to obtain powders of various materials for forming an electrode for electric discharge surface treatment.
For example, titanium (Ti), which is light in weight, high in strength, and less likely to be oxidized at high temperatures, is used for a compressor of a jet engine. A solid of Ti is hardly oxidized except for a portion of a surface being slightly oxidized in an atmosphere, while a portion inside remains as Ti. However, in Ti in a form of powder of which the grain diameter of several μm, an influence of a surface area with respect to volume is increased. Thus, heat generated due to oxidation on a surface of grains of the powder propagates into a portion inside the grains, thereby causing oxidization also in the portion inside of the grains. When oxidized, conductivity, which is a property that the Ti powder originally has, is lost. Therefore, such a powder cannot be used for an electrode for the electric discharge surface treatment. This is because if the electrode does not have conductivity, it is impossible to discharge electricity. Moreover, oxidation of the Ti powder may explosively proceed. For the above reasons, it is difficult to obtain the powder having an average grain diameter suitable for manufacturing an electrode for the electric discharge surface treatment. Even if such powder is available, it is impossible to manufacture an electrode for electric discharge surface treatment by the conventional methods.